Transcript Servlet Overview

Java + XML = JDOM
by Jason Hunter
with Brett McLaughlin
Sun HQ Briefing
January, 2001
Introductions
Jason Hunter
[email protected]
CollabNet
http://collab.net
http://servlets.com
Author of
"Java Servlet Programming"
(O'Reilly)
Introductions
Brett McLaughlin
[email protected]
Lutris Technologies
http://enhydra.org
http://www.newInstance.com
Author of "Java and XML"
(O'Reilly)
What is JDOM?
•
JDOM is a way to represent an XML document for
easy and efficient reading, manipulation, and writing
– Straightforward API
– Lightweight and fast
– Java-optimized
•
Despite the name similarity, it's not built on DOM or
modeled after DOM
•
An open source project with an Apache-style license
– 950 developers on jdom-interest (high traffic)
– 800 lurkers on jdom-announce (low traffic)
The JDOM Philosophy
•
JDOM should be straightforward for Java programmers
– Use the power of the language (Java 2)
– Take advantage of method overloading, the
Collections APIs, reflection, weak references
– Provide conveniences like type conversions
•
JDOM should hide the complexities of XML wherever
possible
– An Element has text content, not a child Text node
with content
– Exceptions should contain useful error messages
– Give line numbers and specifics, use no SAX or
DOM specifics
More JDOM Philosophy
•
JDOM should integrate with DOM and SAX
– Reads and writes DOM docs and SAX events
– Can use any DOM or SAX parser (uses adapter model
or can use JAXP)
•
Why not use DOM:
– Same API on multiple languages, defined using IDL
– Foreign to the Java environment, Java programmer
– Fairly heavyweight in memory
•
Why not use SAX:
– No document modification, random access, or output
– Fairly steep learning curve to use correctly
•
JDOM is to DOM/SAX as RMI is to CORBA
– Java optimized, plays well with the other
Package Structure
•
JDOM consists of four packages
org.jdom
org.jdom.adapters
org.jdom.input
org.jdom.output
The org.jdom Package
•
These classes represent an XML document and XML
constructs:
– Attribute
– CDATA
– Comment
– DocType
– Document
– Element
– Entity
– Namespace
– ProcessingInstruction
– (PartialList)
– (Verifier)
– (Assorted Exceptions)
The org.jdom.input Package
•
Classes for reading XML from existing sources:
– DOMBuilder
– SAXBuilder
•
Also, outside contributions in jdom-contrib:
– ResultSetBuilder
– SpitfireBuilder
The org.jdom.output Package
•
Classes for writing XML to various forms of output:
– DOMOutputter
– SAXOutputter
– XMLOutputter
•
Also, outside contributions in jdom-contrib:
– JTreeOutputter
General Program Flow
•
Normally XML Document -> SAXBuilder ->
XMLOutputter
XML Document
Direct Build
XMLOutputter
JDOM Document
SAXOutputter
SAXBuilder
DOMBuilder
DOMOutputter
DOM Node(s)
The Document lifecycle
•
Documents are represented by the
org.jdom.Document class
– A lightweight object holding a DocType,
ProcessingInstructions, a root Element
(containing other Elements), and Comments
•
A document can be constructed from scratch:
Document doc = new Document(
new Element("root").setText("hi"));
•
Or it can be constructed from a file, stream, or URL:
SAXBuilder builder = new SAXBuilder();
Document doc = builder.build(url);
•
Then it can be output as XML, or SAX, or DOM:
XMLOutputter outputter = new XMLOutputter();
outputter.output(doc, System.out);
Ensuring Well-Formedness
•
The Element constructor (and all other object
constructors) check to make sure the element is legal
– i.e. the name and content don't contain
inappropriate characters
•
The add and remove methods also check document
structure
– An element may only exist at one point in the tree
– Only one value can be returned by getParent()
– No loops in the graph are allowed
– Exactly one root element must exist
JDOM Namespaces
•
Namespace prefix to URI mappings are held statically
in the Namespace class
•
They're declared in JDOM like this:
Namespace xhtml = Namespace.getNamespace(
"xhtml", "http://www.w3.org/1999/xhtml");
•
They're passed as optional parameters to most
element and attribute manipulation methods:
List kids = element.getChildren("p", xhtml);
Element kid = element.getChild("title", xhtml);
Attribute height = element.getAttribute(
"height", xhtml);
The Push to 1.0
•
Currently JDOM is at Beta 5
•
95% of XML vocabularies compliance
– Some work to be done for IDs and IDREFs
– Discussion about Namespace re-factoring
– Inline DTDs still in progress
– In-memory validation and well-formedness in
progress
•
Speed and memory optimizations yet to be done
Extending JDOM
•
Some possible extensions to JDOM:
– XPath (already quite far along, and usable)
– XLink/XPointer (follows XPath)
– XSLT (natively, now uses Xalan)
– JAXP
API Standardization
•
JDOM is being submitted as a Java Specification
Request (JSR)
– Hoping for inclusion in JAXP 1.2+ (or 2.0+)
•
Possible standardization by OASIS or related technical
ratification group
Get Involved
•
Download the software
– http://jdom.org
•
Read the docs
– http://jdom.org
•
Sign up for the mailing lists (see jdom.org)
– jdom-announce
– jdom-interest
•
Java and XML, by Brett McLaughlin
– http://www.oreilly.com/catalog/javaxml
•
Help improve the software!